1. Field of Invention
This invention relates to a fusing device for use with image forming apparatuses, such as copiers and printers, that employ the xerographic technology. More particularly, this invention relates to a toner image fusing device of the type in which a pressure roller or the like is cooled with fans or the like.
2. Description of Related Art
In most apparatuses, such as electrophotographic copiers and laser printers, that adopt the xerographic technology to produce recorded sheets, a toner image formed on the surface of a photoreceptor drum is transferred onto a transfer sheet, and the transfer sheet carrying the toner image is guided to a fusing device by means of a transport belt or the like. In the fusing device, the toner image is fused onto the transfer sheet to complete a recorded sheet.
This type of conventional toner image fusing device is most commonly of a heating roller type as taught in Japanese Utility Model Application Unexamined Publication No. Hei. 2-136273 ("Toner Image Fusing Device in Image Forming Apparatus"). In this heating roller type fusing device, a pressure roller having a metal pipe the surface of which is covered with an elastic member made of, for instance, silicone rubber is pressed against a heating roller having a built-in heater (heat generating lamp) at a specified pressure. And a transfer sheet is nipped between the two rollers so as to receive sufficient heat and pressure. Thus, a toner image is fused onto the transfer sheet.
In some cases of its operation, the fusing device must handle transfer sheets that varies in size over a broad range of, for instance, A0 to A3. The following problem occurs in such a case. When a long and narrow transfer sheet is nipped between the pressure and heating rollers, the portions of the pressure roller over which the transfer sheet does not pass (hereunder referred to as "non-sheet-passing portions") receive heat from the heating roller by the direct contact. As a result, the temperature of the non-paper-passing portions of the pressure roller becomes higher than that of the portion over which the transfer sheet passes (hereinafter referred to as "paper-passing portion"). If fusing of a toner image onto a smaller size transfer sheet is immediately followed by fusing onto a larger size transfer sheet, the above-described uneven temperature profile will cause wrinkles in the transfer sheet or a disorder of a fused image.
For example, Japanese Patent Application Unexamined Publication No. Sho. 48-74235 and Japanese Utility Model Application Unexamined Publication No. Sho. 62-104264 disclose methods for preventing the occurrence of wrinkles in a transfer sheet and other problems by cooling, while the transfer sheet is running, the pressure roller with cooling fans so that its temperature will not elevate unduly. However, this approach is not capable of fully efficient cooling when it is necessary to handle various sizes of transfer sheets coming in a mixed manner.
The occurrences of wrinkles in a transfer sheet and a disorder of a fused image are not limited to the case where fusing of a toner image onto a smaller size sheet is immediately followed by fusing onto a larger size sheet. But the same problems will also occur when the temperature of the environment in which the main body of a copier, a printer or the like is installed is high, or when a transfer sheet is fed from paper set on the machine which paper is in a high-temperature state. In particular, the problems such as wrinkles in a transfer sheet are more likely to occur when the cycle is repeated which consists of causing one transfer sheet to run, ejecting said sheet and, immediately thereafter, causing the next transfer sheet to run.